1. Field of the Invention
This invention pertains generally to pulse-width modulators, and more particularly to a pulse width modulator that has one cycle response, does not require a resettable integrator in the control path, and has nearly constant switching frequency.
2. Description of the Background Art
Switching converters are most often controlled with pulse-width modulation (PWM). PWM is generally realized by comparing a modulation signal with a sawtooth signal. Linear feedback has traditionally been used to achieve control over certain state variables in the switching converters. However, nonlinear control of PWM switching converters has shown excellent improvement compared to linear feedback methods at optimizing system response, reducing the distortion, and rejecting power supply disturbances. These techniques improve performance over the linearly controlled techniques by directly controlling the switching variables (e.g. switched voltage or current) rather than correcting the error after it has occurred. U.S. Pat. No. 5,278,490, which is incorporated herein by reference, describes several approaches that ensure the switching variable exactly equals the control reference each switching period and thus have one cycle response. The constant frequency approach in U.S. Pat. No. 5,278,490 is simple, robust, and has been applied to high fidelity class-D audio applications as shown, for example, in U.S. Pat. No. 5,617,306 which is also incorporated herein by reference. This technique requires a fast reset circuit and integrator to minimize signal distortion during the reset period. The constant on-time version of U.S. Pat. No. 5,278,490 and the method described in U.S. Pat. No. 3,659,184 which is further incorporated herein by reference are also simple techniques providing one cycle response by ensuring the error between the switched-variable and the control reference is zero each cycle. A nice feature of these techniques is that they do not need a re-settable integrator, reducing component speed requirements. However, their switching frequency is widely changing (over 10 times), which deteriorates performance.
Class-D power amplifiers for high fidelity audio amplification require wide bandwidth and low distortion, which imposes a great challenge to the conventional PWM method. Because of the general applicability of linear feedback to the control of many different dynamic systems, it is not surprising that it has also been applied to control PWM amplifiers. However, due to the non-linear nature of switching circuits, one cycle control has several distinct advantages when considering the design of the DC power supply and the switching amplifier power stage. Linear feedback methods are susceptible to the power supply ripple, dead time control, and non-ideal switching edges causing distortion. Therefore, to alleviate these problems, the power source is designed to stringent requirements adding cost, complexity, and weight to the system. Furthermore, it is difficult to apply soft switching methods because they induce resonant switching edges which also add distortion to the system. Since the constant frequency one cycle response methods ensure that each cycle the average value of the switched-variable equals the control reference, they inherently reject power supply disturbances and non-ideal switching edges, dramatically lowering the power source regulation requirements and easily allowing soft switching to be used. For one cycle control methods with widely changing switching frequencies, these advantages are not as prevalent since the frequency modulation effect induces distortion.